Run Docs Agent through the native executor

[chubes4]

Summary

• replace the legacy Docs Agent consumer path with the reusable native Docs Agent workflow
• add a source-level producer contract to CI without coupling it to the generic smoke suite
• pin Docs Agent Bootstrap developer documentation surface #141 and WP Codebox v0.12.20 for sanitized tool-lifecycle observability

Supports Automattic/docs-agent#140.

How to test

1. Run composer install --no-interaction --prefer-dist.
2. Run composer test.
3. Check out Automattic/docs-agent at 662ebbfcfe929673fd8b260c685fbc9dc5807842.
4. Check out Automattic/wp-codebox at v0.12.20.
5. Run DOCS_AGENT_DIR=/path/to/docs-agent WP_CODEBOX_DIR=/path/to/wp-codebox php tests/docs-agent-native-workflow-contract.php.
6. Run actionlint .github/workflows/*.yml.

AI assistance

• AI assistance: Yes
• Tool(s): OpenAI openai/gpt-5.6-sol via OpenCode
• Used for: Advanced the producer pins, updated obsolete contract expectations, and verified the exact producer chain with Chris.

[chubes4]

Superseded by #432, which ports the producer-pin update onto current main and advances Docs Agent to #143 / WP Codebox v0.12.21. #432 passed Workflow contracts, PHP smoke tests, and PHPStan before merge.